This invention relates to methods of making optical quality coatings of inorganic oxides on glass or equivalent substrates, such as metals or ceramics. These inorganic oxide coatings are primarily electrochromic, but may also have other desirable properties such as electrical conductivity and anti-reflection. Electrochromic films undergo reversible coloration induced by an applied electric field or current. These inorganic electrochromic layers can be broadly classified into those that color cathodically due to the double injection of electron and cation (Group VI-B oxides such as WO.sub.3 and MoO.sub.3) and those that color anodically (Group VIII oxides such as IrO.sub.2, Rh.sub.2 O.sub.3, NiO and CoO). Electrochromic coatings are used in information display devices, solar control windows and light modulators.
In a typical electrochromic device, the electrochromic coatings are in contact with an electron conductor and an ion conductor. The electron conductor can be a paste or coating on a substrate, or a stand-alone monolith. The ion conductor, or electrolyte, may be a liquid, paste or solid. Electrochromic coatings work by the injection or ejection of ions and electrons between the electron conductor and the ion conductor.
The most common way to deposit electrochromic films is by vacuum techniques, typically evaporation or sputtering. Non-vacuum techniques such as anodization and atmospheric chemical vapor deposition are also used. Evaporation deposition and sputter coating require a high vacuum. While such techniques require expensive capital equipment, they have been commonly used to produce electrochromic coatings.
Three similar non-vacuum coating techniques which have been used to a limited extent for electrochromic coatings are dip coating, spray coating and spin coating. These wet chemical solution coating techniques offer the advantage of being less capital intensive and thus less expensive. Dip coating, as an example, is commonly used to coat glass with SiO.sub.2. This process involves lowering a glass substrate into a solution containing an appropriate precursor of the desired oxide. Spin coating and spray coating are similar to dip coating except that instead of dipping the glass, the precursor solution is applied to the glass, which is spun to spread the coating out, or is sprayed onto heated glass.
It is desirable to be able to achieve amorphous electrochromic coatings that are resilient to ionic intercalation such as occurs when ions such as hydrogen ions, lithium ions, sodium ions and the like are inserted and removed during electrochromic coloring and bleaching. It is also desirable to achieve amorphous electrochromic coatings that are durable to mechanical abrasion, chemical attack (as for example by acidic electrolytes) and the like.
One way to achieve such coatings is to consolidate the amorphous structure by heating to high temperatures and in some cases even crystallizing the tungsten oxide. Unfortunately, the less expensive wet chemical deposited films above a certain thickness (2,500 angstroms or thereabouts) usually crack during consolidation and crystallization due to volumetric shrinkage, among other effects. While thinner wet chemical deposited films are not as likely to crack during consolidation or crystallization, they do not color as deeply as thicker films.
In addition, when crystallization occurs in wet chemical deposited films, the coloration kinetics of the film becomes slower and/or its ability to color reduces. This is due to a decreased ionic diffusion and may also be related to the potential loss of coloration sites.